Video display monitors are important accessories in a computer system, and only those displays with accurate color characteristics can provide accurate video information. Because of the difference in physical and electrical characteristics among vdms of even the same type, each vdm needs color test and adjustment under the environment of specific light.
The color characteristics of a vdm are the chromaticity of white points and the nonlinearity between the input voltage of a CRT and the output brightness thereof, both can be separately measured.
The RGB values of a color shown on a vdm is determined by the XYZ values of that color through the transformation A.sub.C as described in (1): ##EQU1## where RGB are the values of red, green, and blue colors, representing the colors shown on the vdm, and XYZ are tristimulus values of a color, determined by the chromaticity of a color and its associated white value. Matrix A.sub.C is determined by the chromaticity of both phosphers and white point C. Usually the chromaticity of phosphers is fixed when manufacturing a vdm, therefore it is the white point that determines the RGB of a color displayed thereon. The value of a white point can be adjusted by circuit hardware or controlled by software.
The chromaticity of a white point means the chromaticity of a color viewed to be white. Usually the white point of a display is the chromaticity with which RGB are adjusted to be of equal value. A white point is a visual white color, or called "neutral gray", and means that no any visual color component at all; however, a white point on a vdm will be affected by environmental lights, therefore it is unnecessarily a white color or neutral gray even the RGB of the vdm is set to be of the same value.
Conventional schemes for measuring the chromaticity of a white point on a vdm is by using optical meters such as calorimeter, photometer, or spectroradiometer to measure the output of a display. Although the measurement by optical meters is accurate, it must rely on not only expensive and adjusted instruments, but also specialized knowledge and an environment suitable for performing necessary measurements, thereby it is usually beyond a user's capability to do necessary adjustments or measurements by himself.
Engeldrum et al suggest, in U.S. Pat. No. 5,638,117, another scheme in which a display is used to show a sequence of pictures for users to do adjustments based on matching card. Although the scheme saves the need of measurement by optical instruments, it requires matching card, thereby it is accompanied with higher cost and the inconvenience arising from a loss of the matching card, hence it is to be improved.
On the other hand, the nonlinearity characteristic of a display, which represents the relation between input voltages and displayed brightness, and which can be described as a power function (brightness=[voltage+.alpha.].gamma.), usually requires measuring the voltage and brightness of single points by optical instruments. The relation is determined by parameters .alpha. (called "black level error") and .gamma. (characterizing nonlinearity).
There are two representation modes for nonlinearity characteristic of a display: parameter mode and listing mode. As described above, the parameter mode is based on the values of .alpha. and .gamma.. The listing mode is based on the value of voltage and brightness of multiple points. Both the modes require measuring voltage and brightness of single point by optical instruments, thereby are accompanied with the same drawbacks arising from the usage of optical instruments as described above.
One more point to be considered is that the application of optical instruments is based on hardware-based approach while color sensing by human being is based on the intuition of mankind. It shall be clear the measurement based on human being intuition instead of instrument is better than any way else because the judgment or evaluation on colors eventually relies on the sense of human being.